As an output apparatus for computers and work stations, electrophotographic image-forming apparatuses are known which form an image on a recording medium by using a powdery developing agent (toner). Such an image-forming apparatus is equipped with an image-bearing member like a photosensitive drum on which an electrostatic latent image is formed, and a developing apparatus for supplying a developing agent onto the electrostatic latent image formed on the photosensitive drum.
A conventional developing apparatus is explained below by reference to FIGS. 10 and 11.
FIG. 10 is a schematic drawing showing an example of conventional developing apparatus. FIG. 11 is a front view of a developing apparatus provided with a conventional magnetic sealing member.
The developing apparatus 10 is equipped with a toner container (developing agent container) 12 for holding a magnetic developing agent (hereinafter referred to as “magnetic toner T”), a stirring member 14 for delivering the magnetic toner T with stirring, and a roll-shaped (cylindrical) developing sleeve (developing-agent holder) 16 for holding the magnetic toner T delivered by the stirring member 14 on the peripheral face thereof. The developing apparatus 10 is further equipped with a developing blade 18 for controlling the thickness of the toner held on the peripheral face of the developing sleeve 16 as desired, and a magnetic sealing member 20 for preventing leakage of the magnetic toner T from the toner container 12.
The aforementioned toner container 12 is capable of containing a certain amount of the magnetic toner T. The stirring member 14 is fixed rotatably to the toner container 12, and rotates in the arrow A direction, thereby stirring and loosening the magnetic toner T mechanically and delivering the magnetic toner T smoothly to the developing sleeve 16 to be held thereon.
The developing sleeve 16 is fixed rotatably to bearings (not shown in the drawing) provided on the both sidewalls of the toner container 12. In the hollow in the developing sleeve 16, a magnetic roller 22 is fixed coaxially with the developing sleeve 16. The magnetic roller 22 is magnetized symmetrically to have plural magnetic poles (magnetic poles: N1, S1, N2, and S2) along the periphery direction. Thus, the developing sleeve 16 holds the magnetic toner T by the magnetic force of the magnetic poles N1, S1, N2, and S2, and delivers the magnetic toner smoothly onto an image-bearing member (not shown in the drawing).
The developing blade 18 is placed with a prescribed clearance to the peripheral face of the developing sleeve 16 to hold the toner T in a uniform thickness on the peripheral face of the developing sleeve 16.
The magnetic sealing member 20 is placed at respective ends of the developing sleeve 16 in the length direction (rotation axis direction) at a prescribed interval in a shape of an arc to prevent leakage of the toner T through a gap at the bearing portion (hereinafter called “sealing”).
In a known technique of prevention of leakage of the magnetic toner T through the gap at the bearing portion, an elastic sealing member composed of an elastic material like a felt or a foamed rubber is pressed against the peripheral face at the respective lengthwise ends of the developing sleeve 16 to stop the gaps.
However, with this technique of using an elastic sealing member, the peripheral speed of the developing sleeve 16 can be made irregular by counteraction to the rotation torque by friction between the elastic sealing member and the developing sleeve 16. This irregularity or variation of the peripheral speed may impair the uniformity of toner holding ability of the developing sleeve 16. The lack of uniformity in the toner holding ability may cause non-conformation of the density of the toner image formed on the image-bearing member surface with the image information of the original document to lower the quality of the image formed on a recording medium like a recording sheet.
Therefore, to prevent the lowering of the image quality by use of the elastic sealing member, the aforementioned magnetic sealing member 20 is employed.
The magnetic sealing member 20 is an arc-shaped magnetic body placed near the peripheral face of the developing sleeve 16 with a prescribed gap (0.2–0.8 mm). Thereby a magnetic circuit is formed from by the magnetic sealing member 20 and the magnetic roller 22 in the developing sleeve 16. This magnetic circuit forms a magnetic brush of the developing agent. This magnetic brush prevents the leakage of the developing agent. The magnetic sealing member 20 is placed outside each of the lengthwise ends of the developing blade 18 as shown in FIG. 11.